1. Field of the Invention
This invention relates to a combustion appliance equipped with a safety device which is adapted to suspend the supply of fuel to a burner means of the combustion appliance not only when the oxygen concentration of ambient air has dropped so as to cause an oxygen insufficiency but also before incomplete combustion develops owing to a clogging of a primary air orifice of the burner means or a linting of fins, for example of a water heater, and to thereby prevent carbon monoxide poisoning.
2. Description of the Prior Art
The hitherto-available safety devices for combustion appliances include a flame failure detecting device for preventing leakage of raw gas and an oxygen insufficiency safety device which has recently been a subject of great interest. The general principle underlying said flame failure detecting device is to detect the presence or absence of a pilot flame with a thermocouple and, in this field of art, a variety of improvements have been proposed. Oxygen insufficiency safety devices are also available which, through various burner constructions, are designed to detect a reduction of oxygen concentration of the air from a change of flame temperature before the emission of carbon monoxide gas takes place from a main burner as the oxygen concentration decreases. Although the above devices are capable of precluding evolution of carbon monoxide gas because the combustion range of the burner is restricted in response to a reduction of the oxygen concentration of ambient air, they fail to deal effectively with situations in which the appliance after having remained unused for long has been locally clogged with dust, oil or the like or the burner orifices have been clogged.
One solution proposed to the above problems is a method of suspending the supply of fuel gas in such a manner that when, for example, the fins of a heat exchange device such as a water heater have been linted, the pressure gain within the heat exchanger is fed back to the primary air orifice of the pilot burner so as to dilute the primary air with the products of combustion in the heat exchanger and the resultant change of the pilot burner flame prior to cause evolution of carbon monoxide due to linting of the fins is detected so as to stop the supply of fuel.
This method is able to deal with the problem associated with linted heat exchanger fins but is unable to deal successfully with such problems as the clogging of the primary air orifice of the burner.
Another device previously proposed and capable of dealing with the problem of oxygen insufficiency or a clogged primary air orifice is one incorporating a combustion sensor which utilizes an oxygen ion conductive solid electrolyte (for example, zirconium oxide). This combustion sensor includes an electrode disposed on either side of the sensor and is adapted to generate an electromotive force between the two electrodes when a difference of oxygen concentration develops between the two sides of the sensor.
In order that such a sensor may function as a system responsive to oxygen reduction or a clogging of the primary air orifice, the burner is preferably such that the flame length is short and that all the air required is supplied in a premixed form, i.e. the burner is of the total premixed combustion type. Since such a total premixed combustion burner is so designed that, in normal combustion, an amount of air in excess of the theoretical air amount is usually supplied as primary air, the combustion product gas still contains a small percent of oxygen and it is also so designed that, by the provision of one electrode on this combustion product side and the other electrode on the atmospheric side, only a small electromotive force will be generated between the electrodes. Then, when the amount of oxygen relative to that of fuel becomes in short supply at times of oxygen reduction or at the time of clogging of the primary air orifice, there is available an oxygen concentration of only about 10.sup.-10 at the electrode on the combustion product side and an oxygen concentration of about ten and same percent on the atmospheric side and, due to this difference in oxygen concentration, a large electromotive force is generated between the two electrodes. The generation of this electromotive force is sensed to detect an occurrence of oxygen insufficiency or a clogging of the primary air orifice. However, since the device is such that it detects an oxygen concentration gradient between the electrodes, the choice of positions of the sensor components with respect to the flame is limited. Thus, in order to position one electrode on the combustion product gas side and the other on the atmospheric side, the flame must preferably be short and even. Moreover, while the device is suitable for sensing if all the air requirement is being met by primary air, that is to say if the current combustion is total premixed combustion, it is disadvantageous, when the flame is long and tends to sway as in the case of a busen flame, in that because the sway of the flame becomes large at times, the electrode opposite to the one on the combustion product gas side is brought into a condition similar to that of the on the combustion product gas side so that at the time of oxygen reduction or at the time of clogging of the primary air orifice, the difference in oxygen concentration between the two electrodes is reduced to almost nil and, hence, no electromotive force is generated therebetween. The net result is that the device fails to detect the abnormality.